Transcript Biogeochemical Cycles

Soil Biogeochemical Cycles
Carbon, Nitrogen, Phosphorus
24/118 required by organisms
Macronutrients: C,H,N,O,P,S
Micronutrients
BIOGEOCHEMICAL CYCLES
The complete pathway that a chemical
element takes through the biosphere,
hydrosphere, atmosphere and lithosphere.
Soil Carbon Cycle
CARBON CYCLE
atmosphere
respiration
photosynthesis
biosphere
Plant residues
Applied organic materials
GAINS
Soil organic carbon
Respiration
LOSSES
Plant removal
Erosion
Pools (compartments) of soil organic matter:
(categorized by susceptibility to microbial respiration)
1. Active/Fast
C:N 15:1 – 30:1
1-2 years
readily accessible to microbes; most of mineralizable N
10 – 20% of total
2. Slow
C:N 10:1 – 25:1
15-100 yrs
food for autochthonous microbes ; some mineralizable N
3. Passive
C:N 7:1 – 10:1
500-5000 yrs
colloidal; good for nutrient and water-holding
60 -90% of total
Soil management may help curb greenhouse
effect due to carbon dioxide emissions
pre-Industrial Revolution: 280 ppm CO2
post: 370 ppm
0.5% increase per year
Causes:
1. Fossil fuel burning
2. Net loss of soil organic matter
By changing balance between gains
and losses, may limit loss of OM…how?
How?
1. Restore passive fraction in soils that are
degraded.
-sequesters carbon for long time
2. Switch to no-till practices
3. Convert to perennial vegetation
• Cornfield in warm, temperate climate
Net loss of carbon
Soil Nitrogen Cycle
• Atmosphere 78% nitrogen
• Not in directly accessible form for
organisms
– Made usable by fixation
• Most terrestrial N is in the soil !
– 95-99% in organic compounds
– Made usable by mineralization
Let’s look at all components and processes
in nitrogen cycle…..
A. Nitrogen fixation
1. Atmospheric: lightning
– Oxidation of N2
2. Industrial
production of N fertilizer
N2 + H2 → NH3
3. Biological (soil organisms)
(industrial fixes 85% as much N as organisms)
Biological fixation
(soil organisms)
Immobilization: microbes convert N2 to
N-containing organic compounds
Nitrogenase
2 groups of N-fixing
microorganisms
A. Nonsymbiotic, autotrophic:
(use solar energy)
1. Some actinomycetes
2. Cyanobacter (formerly known as blue-green
algae)
3. Photosynthetic bacteria
B. Symbiotic, in association with legume
plants
(plants supply energy from photosynthesis)
Rhyzobium
Infect root hairs and
root nodules of legumes
Symbiosis: mutualistic: plants provide
energy, bacteria provide ammonia for
synthesis of tissue
Energy-demanding process:
N2 + 8H+ + 6e- + nitrogenase → 2NH3 + H2
NH3 + organic acids → amino acids → proteins
B. Mineralization
(ammonification)
Heterotrophic microorganisms
Decomposition
Organic N compounds broken down to
ammonia; energy released for
microorganisms to use
Organic N + O2→CO2 + H2O +NH3 + energy
C. Nitrification
Oxidizes ammonia to nitrate; 2 step oxidation
process:
1. Nitrosomonas:
NH3→NO2- (nitrite) + energy
2. Nitrobacter:
NO2-→NO3- (nitrate) + energy
D. Denitrification
Completes N cycle by returning N2 to atmosphere
(prevents N added as fertilizer from being
“locked” in roots and soil)
Requires energy; Reduction of nitrate/nitrite
NO2 or NO3 + energy→N2 + O2
(many steps)
Denitrifying bacteria and fungi in anaerobic
conditions
NITROGEN applied to soil as fertilizer
Phosphorus Cycle
Phosphorous Cycle
 P often limiting factor for plants:
 low in parent materials
 inclination to form low-soluble inorganic
compounds
 After N, P is most abundant nutrient in
microbial tissue
Differs from N cycle
1. No gaseous component
2. N goes into solution as nitrate
– Stable, plant-available
But P reacts quickly with other ions and
converts to unavailable forms
Available P in soil solution:
• as H2PO4- or HPO4-2 ion
• Microbes constantly consume and release
P to soil solution
Unavailable forms of P depend on
soil pH:
• High pH: calcium phosphate CaHPO4
– Stable in high pH
– Soluble in low pH
• E.g., rhizosphere, so plants can get it
– Can be transformed to less-soluble Ca-P form
(apatite)
• Low pH: iron and aluminum phosphates
– Highly stable
– Slightly soluble in low pH
Soil phosphorus cycle in a grazing system
Role of mycorrhizae in P cycle:
Can infect several plants:
Hyphae connect plants ; conduits for
nutrients
Fungi get E from plant ‘s photosynthesis.
Phosphate crisis
Arbuscular mycorrhizae and N
cycle
• Involve 2/3 of plant species.
• Unlike most fungi, the AM fungi get their supply of sugars for energy
and growth from their plant partner and not from the decomposition
of organic matter
• AM fungi thrive on decomposing organic matter and obtain large
amounts of nitrogen from it.
• The fungus itself is much richer in N than plant roots, and
calculations suggest that there is as much nitrogen in AM fungi
globally as in roots.
• Since fungal hyphae (the threads of which the fungus is composed)
are much shorter-lived than roots, this finding has implications for
the speed with which nitrogen cycles in ecosystems.
Biotic regulation vs. synthetic
fetilizers
• Biota capture and store soil nutrients and return them to
plants when they need them.
– When plants need nutrients, they stimulate soil biota to release
the nutrients.
• In biotic regulation, nutrients are held in resistant forms,
not readily lost from soil.
• Synthetic fertilizers cause physiological changes in
plants that make them withhold energy from soil biota.